1. Field of the Invention
The present invention relates to in-line type electron guns and color cathode ray tube (CRT) apparatuses using the same. More particularly, the invention relates to a color cathode ray tube apparatus applied in television receivers, computer displays and the like, and an in-line type electron gun that is used for the color cathode ray tube apparatus and is capable of achieving a good image quality by decreasing the size of the electron beam spot at the periphery of the phosphor screen.
2. Description of the Related Art
FIG. 10 shows the basic configuration of a commonly used color cathode ray tube apparatus used for television receivers and the like. As shown in FIG. 10, a color cathode ray tube apparatus generally is provided with a valve 3 including a face panel 1 and a funnel 2 connected to the rear portion of the face panel 1, and an electron gun 20 housed in a neck portion 2a of the funnel 2. A phosphor screen 5 including three-color phosphor layers arranged in dots or stripes that emit R (red), G (green) and B (blue) light, respectively is formed on the inner surface of the face panel 1. In the valve 3, a shadow mask 6 for controlling positions of arrival of electron beams emitted from the electron gun 20 is disposed opposite to the phosphor screen 5. The shadow mask 6 is an electrode for screening the colors of three electron beams 8R, 8G and 8B corresponding respectively to the colors R (red), G (green) and B (blue) that are emitted from the electron gun 20, and has many electron beam passage apertures. In addition, a deflection yoke 7 for deflecting the electron beams 8R, 8G and 8B emitted from the electron gun 20 in the vertical and horizontal directions is mounted on an outer circumference of the funnel 2 on the neck portion 2a side.
In a color cathode ray tube apparatus having a configuration as described above, the three electron beams 8R, 8G and 8B emitted from the electron gun 20 are deflected in the vertical and horizontal directions by horizontal and vertical magnetic deflection fields generated by the deflection yoke 7, and a color image is displayed on the phosphor screen 5 by horizontally scanning the phosphor screen 5 with a high frequency, while vertically scanning it with a low frequency, via the electron beam passage apertures of the shadow mask 6.
Specific examples of the color cathode ray tube apparatus having a configuration as described above include an in-line type color cathode ray tube apparatus using, as the electron gun 20, an in-line type electron gun that emits, toward the phosphor layers of the phosphor screen 5, three electron beams arranged in a line and including a center beam and a pair of side beams that travel on the same horizontal plane, while using a deflection yoke 7 for generating non-uniform magnetic fields including a pincushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field such that the three electron beams self-converge.
Various types of electron guns can be used as the electron gun for emitting the three electron beams arranged in a line, and one example is the type called BPF (bi-potential focus). In addition, various systems can be used as the system of forming the main lens of the electron gun 20, and one example is the system called a field superimposing type main lens system (e.g., see JP3320103,B).
FIG. 7 shows a BPF electron gun using a field superimposing type main lens. As shown in FIG. 7, the electron gun 20 includes: three cathodes K arranged in a line in the horizontal direction; three heaters (not shown) for heating the three cathodes K, respectively; 1st to 4th grids G1 to G4 that are integrated and disposed in this order from the cathodes K side to the phosphor screen 5 side (the right side in FIG. 7). Each of these grids G1 to G4 is provided with three electron beam passage apertures corresponding respectively to the three cathodes K arranged in a line, or with a commonly used electron beam passage aperture through which the three electron beams pass.
A portion in which the 3rd-2 grid G3-2 and the 4th grid G4 are opposite to each other forms a field superimposing type main lens. This configuration is shown in FIGS. 8A and 8B. FIG. 8A is a perspective view showing a portion of the 3rd-2 grid G3-2 shown in FIG. 7, as viewed from the 4th grid G4 side. FIG. 8B is a perspective view showing a portion of the 4th grid G4 shown in FIG. 7, as viewed from the 3rd-2 grid G3-2 side. As shown in FIGS. 7 and 8A and 8B, the field superimposing type main lens is formed by disposing two tubular electrodes 9 opposite to each other, and disposing a plate-like field correction electrode 10 on each of the tubular electrodes 9 on the sides not facing each other. Generally, each of the tubular electrodes 9 includes: a tubular side wall portion 11; an edge portion 12 that is formed by bending the end of the side wall portion 11 and is opposite to the other tubular electrode 9; and a folded portion 13 that is formed continuously with the edge portion 12 and in parallel with the side wall portion 11 inside the side wall portion 11. On each of the opposite sides of the two tubular electrodes 9, an opening is formed by the edge portion 12 and the folded portion 13. The most common shape of the opening formed on the opposite sides of the two tubular electrodes 9 is an elongated flat-sided oval shaped aperture formed by straight lines and semicircles, as shown in FIGS. 8A and 8B.
When the outer diameter of the neck portion 2a of the funnel 2 is approximately 29 mm and electrodes in each of which three electron beam passage apertures are formed are used to form the main lens, the effective diameter of the main lens generally is represented by the diameter of the electron beam passage apertures and is about 5.0 mm. However, by using the above-described field superimposing type main lens system, it is possible to realize an effective diameter of the main lens of about 8.0 mm.
In the electron gun 20, a voltage of about 170 V is applied to the cathodes K, and the 1st grid G1 is grounded. A voltage of about 600 V is applied to the 2nd grid G2, and a voltage of about 8 kV is applied to the 3rd-1 and the 3rd-2 grids G3-1 and G3-2. A high voltage of 30 kV is applied to the 4th grid G4. Then, the cathodes K and the 1st and the 2nd grids G1 and G2 constitute a three-electrode portion for generating electron beams and forming an object point with respect to. the main lens. The 2nd grid G2 and the 3rd-1 grid G3-1 form a pre-focus lens, and this pre-focus lens serves to pre-focus electron beams emitted from the three-electrode portion. The field superimposing type main lens formed by the 3rd-2 grid G3-2 and the 4th grid G4 focuses the pre-focused electron beams on the phosphor screen 5 eventually, forming an electron beam spot on the phosphor screen 5. When the electron beams are deflected to the periphery of the phosphor screen 5 by the deflection yoke 7, a predetermined dynamic voltage is applied to the 3rd-2 grid G3-2, in accordance with the deflection distance. The dynamic voltage applied to the 3rd-2 grid G3-2 is in a parabolic pattern in which the voltage is lowest when the positions of the electron beams are located at the center of phosphor screen 5 and highest when the electron beams are deflected to a corner portion of the phosphor screen 5. When the electron beams are deflected to a corner portion of the phosphor screen 5, the potential difference between the 3rd-2 grid G3-2 and the 4th grid G4 is smallest, so that the intensity (focusing effect) of the main lens is weakest. At the same time, the effect of a quadrupole lens formed by the 3rd-1 grid G3-1 and the 3rd-2 grid G3-2 is strongest. This quadrupole lens is an electric field lens having a focusing effect in the horizontal direction and a diverging effect in the vertical direction. With the above-described configuration, it is possible, by decreasing the intensity of the main lens, to compensate for a phenomenon in which the distance between the electron gun 20 and the phosphor screen 5 increases and the image point is moved farther. Furthermore, it is possible to obtain a quadrupole lens that corrects deflection aberration resulting from the pincushion-shaped horizontal deflection magnetic field and the barrel-shaped vertical deflection magnetic field of the deflection yoke 7.
In order to achieve a good image quality of a color cathode ray tube apparatus, it has been necessary to decrease the size of the electron beam spot on the phosphor screen, and to form the spot in a uniform shape as close as possible to a true circle on the entire screen. Due to the recent spread of the digital broadcasting using high density pixels, there has been an increasing demand for color cathode ray tube apparatuses for television receivers to have the properties of decreasing the size of the electron beam spot on the phosphor screen and forming the spot in a uniform shape as close as possible to a true circle on the entire screen.
On the other hand, in a color cathode ray tube apparatus incorporating an in-line type electron gun that emits three electron beams arranged in a line, the spot of electron beams arriving at the phosphor screen 5 is elongated laterally (horizontally) in the direction toward the periphery of the phosphor screen 5, as shown in FIG. 9. This phenomenon reduces the resolution of the color cathode ray tube apparatus, resulting in deterioration of the image quality. This phenomenon is due to the non-uniform magnetic fields of the deflection yoke 7 formed to converge the three electron beams arranged in a line on the phosphor screen 5, and becomes more pronounced in areas closer to the periphery of the phosphor screen 5. It also becomes pronounced with an increase in the electric current of the electron beams.
Recently, there has been a trend for increasing the angle of deflection of color cathode ray tube apparatuses for television receivers, as the size of their screens increases and their depth decreases. In addition, the non-uniformity of the deflection magnetic fields has become high, worsening the problem that the electron beam spot is elongated laterally (horizontally) at the periphery of the phosphor screen.
That is, it is apparent that decreasing the horizontal diameter of the electron beam spot at the periphery of the phosphor screen is an effective method for improving the image quality. The most effective method for this purpose is to increase the effective diameter of the main lens. In the case. where a field superimposing type main lens system as described above is used to increase the effective diameter of the main lens, it is common to form the electron gun to be mechanically large for attaining a further increase in the effective lens diameter. This results in the necessity of increasing the outer diameter of the neck portion of the funnel.
In this method, however, it is necessary to design a completely new electron gun, as well as designing a completely new deflection yoke, so that a tremendous amount of cost and time will be required. Furthermore, the power consumption of the deflection yoke increases with an increase in the outer diameter of the neck portion of the funnel, resulting in an increase in the power consumption of monitor sets, television receivers and the like. This presents a disadvantage to consumers and therefore is not preferable.
The present invention has been achieved in order to solve the above-described problems in the conventional art, and it is an object of the present invention to provide an in-line type electron gun using a field superimposing type main lens system that can attain good focusing properties by decreasing the size of the electron beam spot on the entire surface of the phosphor screen without being formed to be mechanically large, and a color cathode ray tube apparatus using the in-line type electron gun.